Monday, April 7, 2014

Why Long Fermentation Times are Important for Ester Formation in Malt Whisky

One fact I noticed during my trip to Scotland was that the average weekday fermentation time at scotch whisky distilleries was about 55 hours, but some went as short as 48 hours while others went as long as 75 (weekend fermentations sometimes reaching 120 hours).

It was generally the case that the larger distilleries used shorter fermentation times while the smaller ones had longer fermentation times (Caol Ila used to be an outlier, but appears to have shortened their fermentation times since replacing their old wooden washbacks with stainless steel a few years ago). In addition to Caol Ila, other distilleries such as Ardbeg have also decreased their fermentation times over the last decade, likely in an effort to increase the output of the distillery. But will shorter fermentation times produce the same kind of spirit?

After doing a bit of reading on the subject, I'm willing to say that the answer is probably 'no'. While shorter fermentation times can extract the same amount of alcohol out of a mash as a longer fermentations, there are other processes that need more time.

Lets begin with what happens during the production of malt whisky.

Malted barley is ground in a mill to produce grist, a mixture of flakes, finer flour, and hulls. This is then added into the mash tun, where it is mixed with hot water to extract the simple sugars from the grain. after soaking for some time, the liquid is drained off and progressively hotter water is added each time, usually three or four times total. The water is rather hot, with the first water added at ~65º C, the second at ~75º C, and subsequent waters are between 85-95º C, which extract the last bits of sugar from the grist and are generally recycled to be used for subsequent first and second waters.

Semi-lauder mash tun at Auchentoshan Distillery

The key point in that process is that while temperatures are high, they are not, unlike mashing done at breweries, heated above 100º C. This means that while the microbial cultures living in the malt are significantly thinned by the heat, they are not all killed.

The sugary liquid from the first two waters is cooled to 18ºC and piped over to the washbacks, where cultured yeast is added. As the dissolved oxygen in the wort is quickly consumed, the yeast begin to grow and divide anaerobically, converting the sugars in the liquid into alcohol, carbon dioxide, and other compounds (for more details, see Whisky Science). Because the yeast is pitched at fairly high concentrations, it can out-compete the remaining residual bacteria for the first 30-40 hours of the fermentation. At that point, the yeast begin to run out of steam as they start to choke on their own waste products - alcohol and heat.

Highly active fermentation at Laphroaig Distillery

While the starting temperature of 18-22º C is a bit below the optimal temperature for yeast, it is necessary to start that low because no whisky distillery I have seen has active cooling systems in its washbacks. This means that the liquid will absorb all of the heat produced by the yeast as they multiply and divide, which, over time, ends up being a lot of heat. After 48 hours, the temperature of the wort can rise by 10-20º C. While wine yeasts are sometimes tolerant up to 32º C, the S. cervisiaie strains used by distilleries will begin to suffer above 25º C or so.

Additionally, the end product of fermentation, ethanol, is toxic to the yeast that produce it. Final alcohol concentrations range from 5-8%, which is approaching the upper limit of survivability for S. cervisiaie. While the yeast will attempt to sequester the alcohol by converting it into esters, this is not a long-term strategy.

Both heat and alcohol end up creating the conditions for autolysis. While you may have heard of this process as something that brewers attempt to prevent, it may actually be an important step in developing the flavors of malt whisky (and champagne). As the yeast become stressed, they begin, in essence, to digest themselves. Cells are exquisitely organized to keep different functions in distinct compartments. When those compartments begin to lose coherence, degradative enzymes are loosed upon the rest of the cell, leading to almost complete breakdown. Large polysaccharides, including the major constituents of the yeast's cell wall, are broken down into smaller mono- and oligosaccharides; proteins are broken down into peptides and free amino acids; triglycerides are broken down into free fatty acids and glycerol.

All of those compounds released during yeast autolysis provide fodder for the bacteria that have been lurking in the background during the initial phases of fermentation. A study by van Beek & Priest (2002) found that bacterial communities, primarily lactic acid bacteria, only begin to thrive after 30-40 hours of fermentation and hit their maximum growth after 70 hours.

van Beek (2002)

While the bacteria are important in and of themselves, the intermediate period between 30 and 50 hours is critical because the yeast begin to defend themselves against the growing bacterial communities. Yeast and bacteria have coexisted for billions of years and yeast have developed a number of defensive mechanisms to suppress bacterial competition for resources. One of these defense mechanisms is the synthesis of acids.

van Beek (2002)

As you can see from the table above, acetic acid concentrations rise almost 10X between 40 and 50 hours. From the previous figure you can see that this is where the bacterial community enters an exponential growth phase and starts to present real competition to the yeast. In response, the yeast produce acetic acid to suppress that growth. As noted above, this is also partially a strategy to reduce the concentration of alcohol by converting it into ethyl acetate, though that never rises above mg/L concentrations. The rise in acetic acid has effectively ceased by 65 hours, at which point the yeast have almost all undergone autolysis and the bacteria are dominant.

The major constituent of the bacterial communities during malt whisky fermentation are strains of Lactobacilli. As the name suggests, these bacteria tend to produce lactic acid. This is the end produce of lactic acid fermentation, which breaks down sugars anaerobically. Why is this important to the flavor of whisky? Lactic acid can form esters, primarily ethyl lactate, which has a creamy or buttery flavor. Additionally, Wanikawa et. al. found that lactic acid bacteria hydroxylate unsaturated fatty acids from yeast, which can be esterified into lactones, which have fruity or coconut odors and flavors. Lactic acid bacteria also continue the process of ester synthesis started by the yeast, producing new acetate derivatives of fusel oils. Additionally, the action of lipases continuing to break down the triglycerides from the yeast to produce free fatty acids, which are then available for esterification and the production of fusel oils is continued from the free amino acids released by yeast autolysis via the Ehrlich pathway, which provide the two necessary raw materials for esterification.

To add to the importance of lactic acid bacteria, Simpson et. al. found that there are differences in the strains of bacteria present in the worts of different distilleries in Scotland. These populations are relatively stable, though they do change to some degree depending on time of year and the types of malt being brought into the distillery. Especially in distilleries with longer fermentation times, these bacterial communities may represent one part of their 'terroir'.

Microorganisms growing on the washbacks at Springbank Distillery

So while yeast may get center stage when it comes to the production of whisky, there are other microorganisms that also play important roles in developing the flavors we associate with the spirit but need more time than is usually given to exert their influence. This does not bode well for distilleries that have reduced their fermentation times over the last decade in an effort to increase output. They may find that this spirit is less complex and flavorful than it was before.

18 comments:

In my experience with brewing ferments, even as long as a month, very little autolysis occurs (based solely on flavor). I've accidentally left beer on the lees for months and had no discernible flavor impacts. Thus, I question that "the yeast have almost all undergone autolysis" at 65 hours, although un-cooled commercial ferments such as you describe will be quite a bit hotter. I would expect well over half the yeast to be still viable after 1-2 weeks.

This doesn't change your conclusions; obviously there is some autolysis, which releases desirable compounds for whiskey and facilitates desirable bacterial action. Additionally, I would think that any yeast that get into the still are going to undergo autolysis during heat-up. In American whiskey (sour mash) some of these compounds would end up back in the next ferment with a fresh charge of lactic bacteria. I wonder if anyone has ever 'sour mashed' malt whiskey?

"The period at about 35 h represented a turning point in the fermentation; the yeasts were beginning to suffer heat (the malt whisky fermentation is not temperature controlled) and starvation stresses, and the bacteria were beginning to grow exponentially."

"Viewed by this procedure, the yeast cells decreased in size with time and collapsed in the later stages of the fermentation (from 60 to 100 h)."

"Finally, the yeast cells collapsed and the bacteria began to die [Fig. 3E (70 hr) and F (100 hr)]"

The washbacks used in commercial distilleries are tens of thousands of liters in size, which is going to have a major impact on heat dissipation due to simple surface area:volume ratios. From what I've read, they genuinely do cook themselves to death.

Biochemical processes need their time to get to the best products. If you only concentrate on getting the alcohol then shorter fermentation can be enough. If you like more fruitiness etcetera, you'll need to be al little more patient.

The second part that the paper doesn't mention is that the lactobacillus and acetobacter don't just create primary flavors….they "seed" the distillate with organic acids. When you have organic acids in an alcohol solution, and you add oxygen and time (precisely what happens in a barrel), you get esters. So acetic acid will combine with isobutanol (commonly found in grain fermentations) to form isobutyl acetate, which tastes/smells like strawberries.

Some shops, like Buffalo Trace, still, to my knowledge, add lactobacillus directly with their yeast pitch. In the old days, many shops would add lactobacillus and/or other bacteria directly to their dona tubs as they are stepping their yeast up.

At our shop, we use wooden fermenters that are never steamed, and allow the yeast strains to consume all the sugars, and then wait two days for the acetobacter and lactobacillus to do their job. You get what brewers would call "flor" covering the fermenters, and the organic acids the bacteria leaves behind after eating dextrins et. al., lend the whiskies strawberry and raspberry notes over time in the barrel. Takes about a year and a half for them to begin to form.

This is why many shops use those wooden fermenters. They aren't even remotely sterile, and are critical for depth of flavor in whisk(e)y that cannot be had with yeast alone.

In high ester rum fermentations, which are frequently rife with butyric acid bacteria, you'll get ethyl butyrate (pineapple) after some time in the barrel.

Another thing to take into account is the dissolved oxygen content of the mash. S. cerevisiae can operate under both aerobic and anaerobic conditions. Under aerobic conditions, it is purely cellular growth while under anaerobic conditions it creates the ethanol + CO2 needed. This is why the aeration at the beginning of pitch is necessary for stable yeast content. I'd imagine that the 30-40 hour mark represents a low enough dissolved oxygen content for Lactobacillus and Acetobacter (markedly anaerobic bacteria) to flourish. Couple this with the flagging S. cerevisiae content, as well as the increased ethanol for Acetobacter and unusable sugars (xylose and sugars only breakable by limit dextrinase), makes it the perfect soup for growth.

Malt whisky mashes also have significantly less dissolved oxygen than brewing mashes. Combined with the higher pitching rates and lack of agitation, the yeast will consume all of the oxygen fairly quickly.

Brilliant post, Jordan! Incredibly enlightening and thought provoking. The importance of wooden wash backs is clear. I wonder if mashing periods have changed in the past, as well as the recent shortenings you describe?

This was a good read, a different interpretation of the impact of fermentation times than I've been exposed to in the past. Whiskey distillations are extremely stressful to yeast due to the low oxygen, and high fermentation temps (which is why no one really top crops as in brewing, relying on donna tubs/dried yeast). However, the ranges quoted (25C and 5-8% abv) I don't think are quite right as stress factors. Distiller's yeast (Type M, MX, and Mauri used by a majority of the scotch industry) are all tolerant of temps quite a bit higher than 25C, working pretty effectively up to 33-35C. The distillery i work at is not cooled, and summer fermentation occasionally hit 35C which definitely has flavor impacts but isn't cooking them to death (the washes still finish bone dry). As you mention, few washbacks are temp controlled and a typical starting temp of ~20C would hit 25C quite soon, I imagine within 15 hours.

I've also never seen mention before that S.cerevisiae is capable of producing acetic acid as a defense mechanism, I've understood the rise in acid to be due to the bacteria that are inherent in barley malt. Is that in the van Beek paper?

The acetic acid concentrations are from the van Beek paper, with commentary from Bryan Davis of Lost Spirits. From what I understand, low pH generally reduces the growth of (certain) bacteria, hence things like the sour mash process to decrease pH during mashing. Both yeast and lactic acid bacteria are doing kind of the same thing, albeit with different types of acid. That reduces spoilage bacteria, while letting the yeast and lactic acid fermenters keep growing.